Engine Cooling problem
1998 Vauxhall Nova 6 cyl Front Wheel Drive Manual

I have a vauxhall nova 1.6 gsi, The problem is the car keeps overheating, when the car is cold it runs mint but soon as the car gets warm it holds back when you put your foot down? it has a new gasket also head skimmed, it sometimes looses water? i don't really know whats happening so some help would be really appreciated, many thanks adam